The present disclosure relates generally to methods for modifying the chemistry in steel. More specifically, the method relates to forming a strain aging resistant steel by using nitride-forming elements, such as boron, to limit the free nitrogen content in high-carbon steel, or to meet a free nitrogen content specification in either high or low-carbon steel.
Steel is generally classified as either high-carbon steel or low-carbon steel. Generally, low-carbon steel contains less than 0.25 percent carbon, and high-carbon steel contains more than 0.25 percent carbon. Low-carbon steel is tough and ductile and is used for screws, nails, automotive body panels, and similar products that do not have critical strength requirements. Toughness is defined as the capacity of the steel to absorb energy without fracturing. Ductility is defined as the ability of steel to undergo permanent changes in shape without fracturing at room temperature.
High-carbon steel is much harder than low-carbon steel due to the presence of both iron carbide (cementite) and iron that form pearlite in the steel. High-carbon steel is used for manufacturing parts that have critical strength requirements, such as shafts, axles, gears, crankshafts, couplings, forgings, rails, railway wheels, and rail axles. High-carbon steel can be heat-treated to high hardness, but loses its toughness and ductility. Hardness is defined as the degree to which the steel will resist cutting, abrasion, penetration, bending and stretching. High tensile strength typically corresponds with high hardness.
One method for making steel products is by refining iron ore into steel. First, iron ore is mined from the earth and transported to a steel mill. The iron ore is then mixed with limestone and coal and heated in a blast furnace. The blast furnace melts the iron ore and removes most of the impurities from the liquid iron, creating a product called pig iron. The carbon content of the pig iron is too high for most steel products, so the pig iron is either further refined in its liquid form or cast into ingots for later use. In either case, the pig iron must be refined to reduce the carbon content and form usable liquid steel. The liquid steel may then be cast into slabs or billets for later processing into steel products.
A downside to this method for making steel products is the expense associated with mining and refining the iron ore into pig iron. One solution is to use direct reduced iron, which is iron ore that has been crushed and refined using large amounts of natural gas. However, the economic attractiveness of direct reduced iron is highly dependent upon the price of natural gas; therefore, direct reduced iron is not always a viable option. Steel mills have found that the use of scrap steel is an attractive alternative to either pig iron or direct reduced iron. The scrap steel is melted into liquid steel using an electric arc furnace or a basic oxygen furnace. The liquid steel can then be cast into slabs or billets. If an electric arc furnace is used, the furnace is charged with the scrap steel, and three graphite electrodes inside the furnace create electric arcs that melt the steel scrap into liquid steel. By contrast, if a basic oxygen furnace is used, the furnace is charged with a mixture of scrap and liquid pig iron, and oxygen is blown into the liquid steel to melt the scrap and purify the steel. The liquid steel is then transferred to a ladle metallurgy furnace (LMF) where the chemistry of the liquid steel is adjusted to meet a steel specification. The liquid steel may then be used to make steel products. The use of scrap steel in an electric arc furnace is typically the most cost-effective method for manufacturing steel products.
A commonly manufactured steel product is steel wire. Steel mills manufacture steel wire by forming a billet, rolling the billet into a steel rod, and then cold drawing the rod to form a wire. Cold drawing is the process of forcing a steel rod though a die to elongate and reduce its diameter. The steel rod is not heated before or as it is being forced through the die, hence the name “cold drawing.” When the steel rod exits the die, the rod is called a wire. The cold drawing process may be repeated using increasingly smaller diameter dies to produce a wire of any diameter. The wire may be heat treated after wire drawing for improved mechanical properties. The wire may then be sold as bulk wire or may be formed into wire products, such as lacing wire or springs.
One problem that occurs when wire is cold drawn is strain aging. Strain aging is a condition where the ductility of the steel is reduced such that the steel becomes brittle, and therefore cracks and breaks rather than bends during the subsequent forming process. There are two main types of strain aging: dynamic strain aging and static strain aging. Dynamic strain aging occurs when the rod is cold drawn to form the wire. Static strain aging occurs between the time when the wire is cold drawn and the time when it is formed into a wire product. Strain aging is a significant problem in the steel industry and has been widely discussed in the prior art.
One of the primary causes of both dynamic and static strain aging is the presence of large quantities of “free nitrogen” dissolved within the steel. All of the free nitrogen dissolved within the steel is present as atomic nitrogen. Experiments have shown that when the nitrogen content of steel is greater than about 80 ppm, the steel will exhibit strain aging, and that strain aging is substantially reduced or eliminated when the nitrogen content of steel is below about 65 ppm, about 60 ppm, and preferably about 50 ppm. On an atomic level, strain aging is caused by the relatively small nitrogen atoms impeding the movement of dislocations within the iron atom matrix as the steel passes though the die. It is widely believed that if the movement of free nitrogen within the iron matrix is reduced or “stabilized,” then dislocations can move easier within the iron matrix and strain aging will be reduced or eliminated. Therefore, a need exists for a method of making steel in which the free nitrogen in the steel is stabilized, thereby increasing the steel's resistance to strain aging.
Table 1 below outlines the nitrogen content of steel by different processes:
TABLE 1TypicalSteel Making ProcessNitrogen ContentBasic Oxygen Furnace Using Pig Iron and Steel Scrap 20-40 ppmElectric Arc Furnace Using Steel Scrap and 50-80 ppmCombinations of Pig Iron and Direct Reduced IronElectric Arc Furnace Using Steel Scrap Only50-100 ppmThus, strain aging is particularly problematic in steel produced using an electric arc furnace because such steel has a higher nitrogen content. This is due, in part, to the use of scrap steel, which has a higher nitrogen content than pig iron or direct reduced iron. As a result, the liquid steel formed from scrap steel has a higher nitrogen content than the liquid steel formed from either pig iron or direct reduced iron. In addition, the electric arc from the electrodes causes molecular nitrogen in the air (N2) to dissociate into atomic nitrogen, which is easily absorbed by the liquid steel. Thus, unless the steel mill implements a nitrogen-reduction process, the nitrogen content of the solidified steel will be even higher than the nitrogen content of the steel scrap used to charge the furnace.
The conventional methods for controlling the nitrogen content of the steel involve the use of additional carbon in the charge, additional oxygen during refining, shielding gases during refining, and/or shrouding equipment during transfer and casting. One of the chemical reactions that occurs in the liquid steel is the interaction of carbon and oxygen, which produces carbon monoxide (CO). Because one of the byproducts of the carbon-oxygen reaction is removal of nitrogen from the liquid steel, the amount of carbon and/or oxygen in the liquid steel can be increased to control the nitrogen content of the steel. The carbon content can be increased by using additional charge carbon, injected carbon, pig iron, or direct reduced iron in the steel. The oxygen content can be increased by adding additional oxygen gas to the liquid steel. In either case, the modified refining process requires more raw materials and/or more expensive raw materials and more processing time than steel production methods in which the nitrogen content is not controlled. An additional method of controlling the nitrogen content of the steel is to use a shielding gas, such as argon, to stir the steel and shield the steel from the nitrogen in the atmosphere. By bubbling the shielding gas through the liquid steel to stir it and covering the surface of the liquid steel with a layer of the shielding gas, the amount of nitrogen the steel absorbs from the atmosphere can be controlled. Moreover, the amount of nitrogen the steel absorbs from the atmosphere can be controlled by using shrouding equipment when transferring the steel between the melting furnace, the LMF, and the caster. Shrouding equipment is also effective at controlling the amount of nitrogen the steel absorbs from the atmosphere when the steel is being cast.
The aforementioned methods for controlling the nitrogen content of the steel are not preferred because they significantly increase the cost of producing the steel and/or substantially reduce the throughput rate of steel production. The use of additional charge and injected carbon and carbon in the form of pig iron and/or direct reduced iron increases the cost of the raw materials. The use of additional oxygen during refining increases the cost of the raw materials and requires additional refining time, thereby decreasing the throughput rate of the steel production. Shielding gases, such as argon, increase the production cost of the steel because the shielding gas is generally not recoverable once used. Finally, the use of shrouding equipment increases the capital cost of the steel mill, which when amortized over the life of the equipment, increases the production cost of the steel. If a method for reducing the nitrogen content of the steel existed, then the aforementioned methods for controlling the nitrogen content of the steel would be unnecessary. In other words, if a method for reducing the nitrogen in the steel existed, then steel could be produced without regard for its nitrogen content and the nitrogen content of the steel could be reduced using the nitrogen reduction method. Consequently, a need exists for a method of reducing the nitrogen content of steel.
The conventional methods of reducing the nitrogen content of steel are also not preferred due to concerns about the costs and/or lowered productivity associated with these methods. Another method for limiting the nitrogen content of steel is to refine the steel using a basic oxygen furnace instead of an electric arc furnace. However, basic oxygen furnaces have two disadvantages: they are more expensive to install and operate and they require a charge comprising a mixture of scrap and liquid pig iron. Another method for limiting the nitrogen content in steel is to use a vacuum degasser on the LMF. A vacuum degasser operates under the principle that the nitrogen in the steel is in equilibrium with the nitrogen in the air above the liquid steel. The vacuum degasser lowers the pressure of the air (i.e. creates a vacuum) above the liquid metal, thereby lowering the partial pressure of the nitrogen in the air and causing the nitrogen in the steel to transfer from the liquid phase to the gaseous phase. However, vacuum-degassing equipment is expensive to purchase, install, and operate. The vacuum degassing process is also relatively time consuming because of the slowness in removing nitrogen from liquid steel under vacuum, which lowers the throughput rate of the steel mill. Consequently, a need exists for a simple, quick, and relatively inexpensive method to reduce the free nitrogen content in steel, thereby creating a strain aging resistant steel.